Water-Energy Nexus in the People's Republic of China and Emerging Issues by Perera Pradeep;Zhong Lijin;

Author:Perera, Pradeep;Zhong, Lijin;
Language: eng
Format: epub
Publisher: Asian Development Bank Institute
Published: 2018-08-15T00:00:00+00:00

Cooling Technologies Used in Thermal Power Plants in the PRC

In thermal power plants using steam turbines (coal-fired power plants, combined-cycle gas turbines with a steam cycle, nuclear power plants, and concentrated solar power plants), the steam from the turbine must be cooled further to achieve maximum thermal efficiency. The choice of cooling systems has the greatest impact on the water requirements of a thermal power plant. With a heat transfer capacity four times that of air, water is a much more effective coolant when it is available in sufficient quantities. Water (wet) cooling systems can be configured as once-through (open-loop) or closed-loop cooling systems. In water-scarce locations, dry (air) cooling and hybrid cooling systems that use a combination of wet and dry cooling are adopted (Box 3) (G.Thopi and A. Pouris, 2016.).

Box 3: Cooling Technologies Used in Thermal Power Plants in the People’s Republic of China

Once-through (open-loop) cooling systems use an ample supply of water from natural water bodies, such as seas, rivers, lakes, or cooling ponds, to pass through the steam condenser (heat exchanger) of the power plant. Water is returned to the water body 10°C–20°C warmer than when it was extracted. Most of the water withdrawn is returned (about 99% of the water is returned; the rest is lost to evaporation), without significant impact on water quality except for an increase in water temperature. Open-loop systems require significant amounts of water withdrawal and are, therefore, vulnerable to droughts, and compete for water with other users. In addition, the discharge of high-temperature water into natural water bodies may affect aquatic ecosystems. In spite of its low cost and low water consumption, open-loop cooling using freshwater is not widespread in the PRC because of a lack of suitable surface water bodies in close proximity to coal-fired power plants. However, in coastal areas, the use of seawater in open-loop cooling systems is common among coal-fired power plants and nuclear power plants. A typical 500 MW coal power plant using open-loop cooling withdraws 0.5 bcm of water per year and consumes 0.0029 bcm per year (Meldrum et al., 2013).

Closed-loop (wet) cooling systems differ from open-loop systems in that the water used for cooling the steam turbine exhaust is recirculated after cooling instead of being discharged into natural water bodies. The heated water is cooled by being passed through a wet cooling tower, cooling pond, or surface water body (e.g., river) before it is recycled. Although water consumption due to evaporation is roughly 10 times higher than that for open-loop cooling systems, water withdrawal is about 100 times lower. In addition, the capital costs of closed systems are 40%–50% higher than the capital costs of open-loop systems. In the PRC, closed-loop systems with cooling towers are preferred because of their lower water withdrawal requirement and environmental impact compared with open-loop systems. A typical 500 MW coal power plant using closed loop wet cooling systems withdraws 0.01 bcm of water and consumes 0.0084 bcm of water per year.

Dry cooling systems are similar to closed-loop wet cooling systems, but use air instead of water to condense the steam.

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